1. Field of the Invention
The present invention generally relates to a negative photosensitive composition sensitive to radiation and a method for forming a resist pattern. More particularly, it relates to a negative photoresist suitable especially for production of semiconductor integrated circuits.
2. Discussion of Background
As generally reported, high integration of semiconductor integrated circuits progresses at a rate of four times in three years. For example, in a case of a dynamic random access memory (DRAM), a device having a memory capacity of 4 megabit DRAM is commercially available by now. Accordingly, the requirements in the photolithography technique indispensable to the production of integrated circuits have become strict year by year. For example, for the production of 4 megabit DRAM, a lithography technique at a level of 0.8 .mu.m is required, and it is expected that for a more highly integrated circuit at a level of 16 megabit or 64 megabit DRAM, lithography at a level of 0.5 .mu.m or 0.3 .mu.m will be required, respectively. Accordingly, it is desired to develop a resist capable of being used for lithography of less than a half micron level.
A negative resist in which cyclized rubber and a bisazide compound as a crosslinking agent are used, is well known as a resist useful for photolithography. However, with such a negative resist, since swelling takes place during development by the action of an organic solvent used as a developer, the resolution of the resist results in 3 .mu.m at best. Therefore, such a negative resist is not suitable for the production of a highly integrated device. Further, the organic solvent used for the development is problematic from the aspects of environmental pollution and health or in view of inflammability.
Further, a positive photoresist comprising a naphthoquinone diazide type photosensitive agent and an alkali-soluble novolak resin is well known. However, such a positive photoresist exhibits a substantial absorption at 300 nm or less and thus has a drawback that when exposure is conducted at a short wavelength, the pattern profile will be very poor. Therefore, exposure has to be conducted at a wavelength of at least about 350 nm, whereby the resolution is limited and it is not useful for lithography of less than a half micron level.
Accordingly, in order to obtain a higher resolution at present, it is conceivable to employ a light in the deep UV region for exposure using a low pressure mercury lamp or excimer laser as a light source, or to employ i-line (366 nm) or deep UV for exposure using an improved reticle with phase shifting method. However, for the phase shifting method, a negative resist is more suitable than the positive resist. Thus, a deep UV resist or a negative resist for the phase shifting method is presently desired.
Japanese Unexamined Patent Publication No. 164045/1987 discloses a negative photoresist composed of a combination of an acid curable resin and a photo-acid-generating material. Specifically, such a photoresist contains methoxymethylated melamine (Cymel.RTM. 303, trade name, manufactured by Mitsui Cyanamid) as a crosslinking agent. However, this photoresist has a problem that the pattern size dependence on the film thickness is substantial due to in-film multiple reflection.
Because of wafer topography, a film spin-coated thereon will have a thick portion and a thin portion. This level of thickness difference of the film did not create any substantial problem with the resolution at a level required heretofore. However, in order to improve the resolution, even this level of thickness difference causes a serious problem. Namely, due to the difference in the thickness of the coated resist film in one wafer which necessarily results in the coating step, the quantity of light absorbed in the film, per area, changes by the influence of the in-film multiple reflection, and accordingly, there will be a problem that the width of the resist line pattern changes. Thus, the influence of the in-film multiple reflection can no longer be ignored.
However, among negative resists for KrF excimer laser or negative resists for i-line (366 nm) phase shifting presently known for lithography of 0.3 .mu.m level, no resist is known to provide a sufficiently small dependence of the width of the resist line pattern on the above mentioned in-film multiple reflection.
On the other hand, Japanese Unexamined Patent Publication No. 15270/1990 discloses a crosslinking agent prepared by alkoxy-methylating a urea derivative, but such a crosslinking agent still has a problem with respect to the resolution.
SUMMARY OF THE INVENTION
It is an object of the present invention to solve the above problems of the prior art and to provide a photoresist excellent in resolution, sensitivity and storage stability, with which exposure can be conducted with a light in the deep UV region and which is also applicable to a phase shifting method and thus to provide a resist excellent for lithography of less than a half micron level.
Another object of the present invention is to provide a photoresist, whose sensitivity dependence on the thickness of the coated film is sufficiently small, i.e. a photoresist in which a variation in the width of the resist line pattern due to the variation of the thickness of the coated film is sufficiently small.
A further object of the present invention is to provide a method for forming a resist pattern using such a photoresist.
The present inventors have conducted extensive studies to solve the above mentioned problems and as a result have found that a photoresist composition composed of a combination of an alkali-soluble resin, a photo-acid-generating material and a certain specific crosslinking agent is a high performance negative photoresist composition which is capable of presenting a non-swellable pattern profile with a high resolution when developed with an alkali aqueous solution after exposure and is excellent in the resolution and storage stability and yet the film thickness dependence of the width of the resist line pattern is small. The present invention has been accomplished on the basis of this discovery.
The present invention provides a negative photosensitive composition comprising an alkali-soluble resin, a photo-acid-generating material and a crosslinking agent for the alkali-soluble resin, which acts under an acidic condition, wherein the crosslinking agent is a compound of the following formula (I): ##STR2## wherein each of A.sup.1 to A.sup.10 which are independent of one another, is a hydrogen atom, --CH.sub.2 OH or --CH.sub.2 OR, wherein R is C.sub.1 -C.sub.4 linear or branched chain alkyl, and X is C.sub.2 -C.sub.10 linear or branched chain alkylene, provided that at least four among A.sup.1 to A.sup.10 are --CH.sub.2 OR, and a plurality of --CH.sub.2 OR may be the same or different.
The present invention also provides a method for forming a resist pattern on a substrate, for a semiconductor which comprises coating the above negative photosensitive composition of the present invention on the substrate, then subjecting it to exposure by a mercury lamp or excimer laser, further subjecting it to heat treatment, followed by developing it with an alkaline aqueous solution to form a resist pattern on the substrate.